I had something similar happen on mine. My issue was the vuk switch. The ball should go in the Ock hole and close that switch solid. Mine would sometimes flutter between closed an opened as the ball would settle. I'd see this in test. I reseated the switch and adjusted the arm and the problem was solved. It'll still happen once in a great while, but it is rare.

Trial and error really. If I remember the problem usually happened when the ball would finally settle and the switch would remain open. I guess since it activated at some point the vuk would kick, but no credit for shot. The wood block can get beat up in there. A major divet or splintering could interfere with the ball sitting properly and not activating the switch consistently. Maybe the switch just needs a slight bend. Not exactly sage advice, but play around with it in test and see if it gets better.

My guess would be to make the switch a little deeper in the hole(bend it down). That way it won't usually trigger until the ball settles. Again, this is just based on what I saw while messing around with mine. Do you see the problem all the time, or just randomly? Works sometimes and not others?

Wow, that is pretty inconsistent. Did attempting to adjust the switch make any difference at all? If you start the game, not in test, and just role the ball in there by hand does it make any difference? I've always gotten the advice that if it works in test and not in game replace it. The switch just might be dodgy.